The present invention relates to a tracking error detecting apparatus for use in a multi-beam optical disk device in which signals are recorded or reproduced by using a plurality of beam spots.
In an optical disk device, a record track on an optical disk is traced with the aid of a light beam spot by moving an optical head in a radial direction of the disk while the optical disk is rotated. In order to trace the record track correctly by the light beam, the tracking control and focusing control are effected.
The tracking control has been carried out by various methods, and a typical push-pull method (sometimes called far field method) will be explained. As shown in FIG. 1a, FIG. 1b and FIG. 1c, in a surface of an optical disk 1 there are formed grooves 2, and a light beam spot is projected on the disk surface by means of an objective lens 3. A distribution of the intensity of light reflected by the disk 1 is shown by P. FIG. 2 illustrates the construction of a tracking error detecting circuit in which output signals from two-divided photodiode 4 having two elements 4a and 4b are supplied to a differential amplifier 5 which then produces a tracking error signal T.
Upon the two elements 4a and 4b of the two-divided photodiode 4 is made incident a light spot S of the light beam reflected from the disk. FIG. 1b shows the case in which the tracking is correctly attained, and intensity of light spot S formed on each of the elements 4a and 4b is equal to each other. Therefore, the tracking error signal T generated by the differential amplifier 5 is zero.
FIG. 1a and FIG. 1c depict the cases in which the tracking is not correctly attained, and the center of the light beam spot is deviated from the center of the groove 2. In this case the intensities of the light spot S reflected from the optical disk 1 and impinging upon the elements 4a and 4b are not identical with each other. Therefore, the differential amplifier 5 generates the tracking error signal T having positive or negative polarity and the tracking error signal T thus obtained is used to perform the tracking control.
The focusing control has been effected in various methods. In FIG. 3, the light reflected by the optical disk 1 is made incident upon a photo detector 8 by means of the objective lens 3 and cylindrical lens 7. The photo detector 8 is divided into four light receiving elements 8a, 8b, 8c and 8d as illustrated in FIG. 4. Output signals Ia, Ib, Ic and Id generated from the elements 8a, 8b, 8c and 8d, respectively are added by adders and then are supplied to a differential amplifier 9 to derive a focusing error signal F=(Ia+Ib)-(Ic+Id).
When the optical disk 1 is in the in-focus position, the light beam reflected by the optical disk 1 forms a circular light beam spot S on the light receiving element 8 as shown in FIG. 5b, so that the focusing error signal F becomes zero. However, when the optical disk 1 is in the out-of-focus position, the reflected light beam spot S having a shape shown in FIG. 5a or FIG. 5c is formed on the light receiving element 8, and thus the focusing error signal F having positive or negative polarity is generated. The objective lens 3 is moved by the automatic focusing control mechanism to attain the correct focus condition.
FIG. 6 is a view showing a whole construction of the above mentioned tracking control and focusing control. In the focusing control system, the focusing error signal F generated by the differential amplifier 9 is amplified by an amplifier 10a and then is supplied to a lens driving mechanism 11f which moves the objective lens 3. In the tracking control system, the tracking error signal produced by the differential amplifier 5 is supplied via an amplifier 10b to a lens driving mechanism 11t to move the objective lens 3. J1 denotes a slider signal for accessing a desired track, and when the optical head is moved in the radial direction to access a desired track, the slider signal is supplied via a driving amplifier 10c to a slider 12. To this end the optical head is placed on rollers 13. The driving amplifiers 10a and 10b have a frequency range of several tens k Hz to effect the fine tracking control, while the driving amplifier 10c has a frequency range of several k Hz to effect the coarse tracking control. A switch 14 is provided to select either the slider signal for retrieval J1 or the tracking signal T, and a switch 15 is provided for selecting one of the tracking signal T and a track jump signal J2.
In order to access a given track on the optical disk by moving the optical head over a long distance, the switch 14 is connected to J1 side and the tracking control system is disconnected. By means of the slider signal J1, the slider 12 is accelerated into a given direction for a given time period and then is decelerated. Next the switch 15 is switched into the side of the track jump signal J2 and the light spot is jumped over one track by supplying a pulse current. An address signal recorded in a track is checked to derive the number of tracks over which the light spot has to be jumped until a desired track is accessed, and the track jump is repeated by the detected number of times. After the desired track has been accessed, the switches 14 and 15 are driven into the side of the differential amplifier 5, and then the usual tracking control is carried out with the aid of the tracking error signal T.
In a multi-beam optical disk device in which a plurality of light beams are projected from a single optical head and a plurality of tracks are simultaneously recorded or reproduced in order to increase the data transfer rate, it is also possible to derive the focusing error signal F and tracking error signal T by similar methods to those explained above.
In order to effect the track jump accurately each time the track access is to be performed in the multi-beam optical disk device, it is necessary to derive tracking error signals for respective light beam spots. For instance, a tracking error detecting circuit of the push-pull method shown in FIG. 7a and FIG. 7b may be used. In FIG. 7a, the two-divided photodiode 4 illustrated in FIG. 2 is used for the four light beams, and in FIG. 7b, a plurality of two-divided photodiodes 4 are used. The function of the device shown in FIG. 7b is substantially same as that of the device depicted in FIG. 7a. A reference numeral 16 denotes an adder.
In the push-pull method, a difference in intensity of first order interference light in the perpendicular direction to the track direction due to the groove 2 of the optical disk 1 is detected, so that a signal similar to the tracking error signal is derived even when the objective lens 3 is shifted in the tracking direction with respect to the optical axis and when the optical disk is inclined. Therefore, the zero point of the tracking error signal is shifted to produce the tracking off-set. That is a fault of the push-pull method. Although the tracking error signal is obtained from a predetermined single light beam, the tracking off-set signal is increased by n times as compared with the tracking off-set which is obtained by a signal light beam, wherein n is the number of light beams which are used simultaneously. In practice, there is obtained a signal which is a mixture of the tracking error signal and the tracking off-set signal and it is difficult to extract only the tracking error signal.